I have used your recipe for grilled prime rib w/garlic & rosemary many times for Christmas with great success. This year, however; I have been asked to make it for my In-Laws whom do not have the appropriate equipment to prepare it at their home. I am going to fix it along with the one for my parents on Sun. the 21st but will not be serving it until Thur. the 25th. Would it be best to grill it completely, freeze it and then simpley rewarm it or partially grill it, freeze it and then finish it on their gas grill the day it will be served?

I cannot speak for the prime rib as I'm doing my first this weekend. (any advise you can pass on in the Holiday Feast Thread would be appreciated).
However myself and others try to limit the ammount of time that meat is frozen, some won't have anything to do with frozen meat. I wouldn't cook and freeze meat, I'm afraid the texture and flavor would suffer.

Partial cooking and then refrigeration may be ok, but I'd prefer and find a way to do it all in one shot.

Well I think that everyone knows my thoughts on frozen meats - and I would certainly not freeze it after it was cooked. I they do not have a grill I would consider cooking in the oven rather than cooking ahead of time and then freezing.

I have to weigh in on this one also. You mentioned they have a gas grill. Go earlier and prepare the meal like you would at your house. Nothing beats the flavor of fresh off the grill. Freezing, while it serves a purpose at times, loses some of the best flavor and juices.

I agree with all previous posts on this one. It sounds as if you are perhaps a bit of a newbie on cooking on the gas grill. This may be an incorrect impression on your post, however, with a little knowledge on the gas grill characteristics, quite good results can be had. ( in fact many people prefer cooking on a gas grill) Look into perhapse using a foil pouch or smoker box, if you have one, to get that smoke flavor you like. I believe that any results cooked on the gas grill will have far superior results than cooking or partially cooking and then freezing. As a second choice- doing it entirely in the oven will still have a much better outocme than partially freezing a partially done roduct.